The MacBook Air will allow you to use the iPhone and the Apple Remote app. (I assume you are using iTunes.)
You might want to take a serious look at the Apple Time Capsule. Not only does it allow automatic and wireless backup with Time Machine, but it can act as a wireless hard drive. This would allow you to have access to a large hard drive without external boxes and cables, and noisy spinning platters which can "tax" the power supply of the Air. What's the use in having solid state storage, if your going to directly connect a "spinning platter" drive? Time Capsules and NASs allow more flexibility and quality when setup correctly. This especially has merit since you are using a USB audio connection. It's usually best to keep the audio connection and storage connection on different protocols.
I would also recommend testing your USB to SPDIF converter to make sure it will be powered correctly by the Air. I don't think your converter requires much, if any, power.
Your thinking is right, you just need to make sure you do it correctly to benefit from what the Air can offer.
You might want to take a serious look at the Apple Time Capsule. Not only does it allow automatic and wireless backup with Time Machine, but it can act as a wireless hard drive. This would allow you to have access to a large hard drive without external boxes and cables, and noisy spinning platters which can "tax" the power supply of the Air. What's the use in having solid state storage, if your going to directly connect a "spinning platter" drive? Time Capsules and NASs allow more flexibility and quality when setup correctly. This especially has merit since you are using a USB audio connection. It's usually best to keep the audio connection and storage connection on different protocols.
I would also recommend testing your USB to SPDIF converter to make sure it will be powered correctly by the Air. I don't think your converter requires much, if any, power.
Your thinking is right, you just need to make sure you do it correctly to benefit from what the Air can offer.